Cellular communications systems continue to grow in popularity and have become an integral part of both personal and business communications. Cellular telephones allow users to place and receive voice calls most anywhere they travel. However, with ever increasing numbers of cellular phone users comes greater challenges for wireless communications device and network providers. One such challenge is addressing interference caused between multiple cellular devices operating in a given geographical area. Cellular devices communicate with a cellular base station using common or shared wireless communications channels (i.e., frequencies). Yet, in some cases signals between other devices and a base station using the same channel may cause a desired signal from the base station to be significantly degraded or even dropped by the handheld device. Such interference is called co-channel interference.
Because of the increasing load on cellular communications infrastructures, various single-antenna interference cancellation (SAIC) approaches have been investigated to meet requirements for Downlink Advanced Receiver Performance (DARP). This effort is being standardized by the third generation mobile communications system and the Third Generation Partnership Project (3GPP).
One SAIC technique that has been investigated is based upon joint demodulation of the desired and interfering sequences. Generally speaking, this approach begins with a standard least-squares (LS) estimate of the propagation channel and a static channel profile for the interferer. Then, a modified Viterbi decoder is used in which half of the state bits represent the user sequence and the other half represent the interferer. A joint branch metric is minimized and the estimated sequences for the desired and interfering signal are used in a least mean squares (LMS) algorithm to update the channel estimates for both the desired and interfering propagation channel.
The 3GPP initiative has given consideration to the application of joint demodulation in synchronized wireless networks. See, e.g., “Feasibility Study on Single Antenna Interference Cancellation (SAIC) for GSM Networks,” 3GPP TR 45.903 Version 6.0.1, Release 6, European Telecommunications Standards Institute, 2004. This is the more limited case that requires one to assume that the base station synchronization data sequences (i.e., training sequences) of the desired-signal and dominant-interferer overlap, which in turn makes the estimation of the CIRs possible using previously known techniques. It also requires one to assume that the interferer will be dominant for the entire burst.
However, in asynchronous network applications the training sequences of interfering signals may not overlap those of the desired signal, which makes CIR estimation problematic. Accordingly, further developments may be desirable to make joint demodulation techniques practical to implement in both synchronous and asynchronous networks.